Disposable absorbent articles such as feminine hygiene products are designed to absorb fluids from the wearer's body, and are well known in the art. Users of feminine hygiene products have several concerns. Leakage from products like catamenial pads, and in particular sanitary napkins, is a significant concern. The feel of the product against the wearer's body is also a concern. Users of such products want the surface of such products to provide a cleaner, more sanitary and drier aspect than common cloth or hydrophilic nonwoven materials have historically provided.
To provide better comfort, current sanitary napkin products are typically provided with a topsheet that is flexible, soft feeling, and non-irritating to the wearer's skin. The topsheet does not itself hold the discharged fluid. Instead, the topsheet is fluid-permeable to allow the fluids to flow into an absorbent core.
Over the years, topsheets have improved to provide a cleaner, drier, and more comfortable in-use experience. In some feminine hygiene products, the topsheet is made of a hydrophobic material. These materials can include phobic nonwovens, hi-loft nonwovens, and softer films with significant texture (micro apertures, nubs that can trap fluid within the film structure etc), and can be manufactured from a wide range of materials such as apertured plastic films, natural fibers (e.g., wood or cotton fibers), synthetic fibers (e.g., polyester or polypropylene fibers) or from a combination of natural and synthetic fibers. The hydrophobic nature of these materials helps to isolate the wearer's skin from liquids absorbed by the product and thus improves comfort by reducing the phenomenon known as “rewet”.
Absorbent cores are well known in the art, and have conventionally included tangled masses of fibers, i.e., fibrous webs that can imbibe fluids both by an absorption mechanism (in which fluid is taken up by the fiber material itself) and by a wicking mechanism (in which fluid is acquired by, distributed through, and stored in capillary interstices between fibers).
One way to improve the absorbent capacity of an absorbent core is to add a superabsorbent material, such as a polymeric gelling material (also referred to as hydrogel-forming material, superabsorbent polymers, etc.) that imbibes fluid, forming a gel that retains the fluid. The superabsorbent material can be provided in particulate form, and encased or enveloped within a tissue layer. Such encased or enveloped cores are often referred to as tissue laminate cores. See, for example, U.S. Pat. No. 4,950,264 (Osborn), issued Aug. 21, 1990 and U.S. Pat. No. 5,009,653 (Osborn), issued Apr. 23, 1991, which disclose tissue laminate cores used in sanitary napkin products.
While superabsorbent materials are quite good at absorbing urine and similar water-based fluids, they absorb blood-based fluids much more slowly (they absorb urine 10 to 12 times faster than they absorb blood-based fluids). Because cellulose fluff—a common core material—absorbs blood-based fluids much more quickly than conventional superabsorbent materials do, relatively high levels of superabsorbent material are needed to provide significant improvements in the rewet, dryness, and absorbing capacity of feminine hygiene products. Conventionally, the concentration of superabsorbent materials in the core of a feminine hygiene product is in the range of 5-200 grams per square meter of material.
At this concentration, larger superabsorbent material particles can act to create a rough, sandpaper-like texture. This effect can reduce the user's comfort, particularly in thin ultra type sanitary pads and pantiliners. In addition, swollen superabsorbent material particles can hinder the fluid movement within an absorbent core by filling the voids in the fiber matrix, thereby restricting permeability of the fiber matrix. When this occurs near the topsheet, it can impair fluid transport to other parts of the product, effectively reducing the capacity of the product and reducing its effectiveness. Accordingly, superabsorbent materials are conventionally moved away from the topsheet in a feminine hygiene product, sometimes being added only or primarily to layers of the core near the backsheet.
Although the properties of a phobic nonwoven topsheet helps to keep fluids in the core, the tendency of such topsheets to repel fluid creates a risk that the fluid will flow off the topsheet, rather than flow through the topsheet and into the absorbent core. This challenge is particularly keen in connection with feminine hygiene products, which commonly encounter fluid flow rates of less than 1-3 gram per hour, rather than the gushing flow rates commonly encountered by incontinence products. For such fluid to flow into a feminine hygiene product, it must overcome not only the hydrophobic properties of the topsheet, but also the natural adhesion of the fluid to bodily surfaces.
To address these concerns, steps are sometimes taken to make the upper surface of the topsheet hydrophilic. This has been done, for example, by treating the upper surface with a surfactant, as described in U.S. Pat. No. 4,950,254. However, treating the topsheet in this way may make it less comfortable against the user's skin.
To help ensure that fluids flow into the absorbent core, some feminine hygiene products with hydrophobic phobic topsheets are constructed with what is sometimes referred to as a secondary topsheet 20 directly beneath the topsheet. This secondary topsheet 20 is designed to acquire the fluid on a liquid-permeable topsheet and distribute it to the underlying absorbent core. To help ensure that the secondary topsheet transfers the fluid to the absorbent core, secondary topsheets are typically made from an air-laid-tissue web or a synthetic nonwoven that has sufficient capillarity to draw the fluid through the topsheet. To ensure that the fluid flow continues on to the absorbent core, the secondary topsheet is commonly designed with more permeability than the absorbent core, and less capillarity than the absorbent core.
While secondary topsheets help to increase the probability of fluid bridging across a hydrophobic topsheet, there remains a need for new and better ways to encourage liquid flow through a phobic nonwoven topsheet and into either an absorbent core or secondary topsheet.
Fluid transport has commonly been achieved by increasing the capillarity of the layer where the fluid is to be directed. This can be done in several ways. For example, densifying the layer where the fluid is to be directed decreases the average pore size in that layer, increasing capillarity of that layer with respect to an undensified layer. Alternatively, a blend of fine fibers and particles that have a high surface area can be added to the layer where the fluid is to be directed. Because area per unit volume has a strong influence on the capillary pressure of a particular substrate, these high-surface-area fibers provide higher vertical wicking in that layer.
However, attempting to increase the capillarity of a secondary topsheet in these traditional ways may cause the secondary topsheet to become a stronger and a better competitor for the fluid, thus impairing transport of the fluid to lower, underlying layers of the absorbent core. As such, there remains a need for absorbent articles having improved absorption characteristics.